1. Field of the Invention
The present invention relates to a polymer sheet for an inner liner and a pneumatic tire using the same.
2. Description of the Background Art
Recently, an attempt has been made to perform weight saving of tires because of strong social demands for fuel economy of automobiles. An attempt has also been made to perform weight saving of inner liners, among tire members, that are disposed at a tire radial inner side and have the function of improving air permeation resistance by decreasing the amount of air leaked to the outside from the inside of pneumatic tires (air permeation amount).
In a rubber composition for an inner liner, an improvement in air permeation resistance of tires is performed by using a butyl-based rubber containing 70 to 100% by mass of a butyl rubber and 30 to 0% by mass of a natural rubber. The butyl-based rubber contains, in addition to butylene, about 1% by mass of isoprene that enables co-crosslinking with an adjacent rubber along with sulfur, a vulcanization accelerator and zinc white. In the case of a conventional formulation, the butyl-based rubber requires a thickness of 0.6 to 1.0 mm for tires for passenger cars, or a thickness of about 1.0 to 2.0 mm for tires for trucks and buses.
In order to perform weight saving of tires, it has been proposed to use, as an inner liner, a thermoplastic elastomer that has excellent air permeation resistance as compared with the butyl-based rubber and can decrease the thickness of an inner liner layer. However, the thermoplastic elastomer that shows high air permeation resistance with a smaller thickness than that of the butyl-based rubber is inferior in vulcanization adhesive strength with an insulation or carcass rubber adjacent to the inner liner as compared with the butyl-based rubber. When the inner liner has low vulcanization adhesive strength, an air-in phenomenon occurs in which air permeates into the space between the inner liner and the insulation or the carcass, and thus a number of small air bubbles appear. This phenomenon is not particularly disadvantageous for tire performance, but raises a problem that a small spot pattern formed inside tires gives an impression of poor appearance to users.
Japanese Patent Laying-Open No. 09-165469 (Patent Document 1) proposes a pneumatic tire capable of improving adhesion between an inner liner and a rubber composition that forms the inner surface of the tire or the carcass layer by forming an inner liner layer using nylon having low air permeability. However, in the technique of Patent Document 1, there is a problem that a rubber cement composed of a rubber composition must be bonded to a nylon film having been subjected to an RFL treatment so as to form a nylon film layer, resulting in complicated steps. Furthermore, in the vulcanization step, there is usually employed a tire vulcanization method in which a bladder body is inserted into an unvulcanized tire (green tire) held in a mold and the bladder body is expanded, and then vulcanization molding is performed by pressing the tire against the inner surface of the mold from the inside of the unvulcanized tire. In the inner liner layer of Patent Document 1, the inner liner layer composed of the nylon film layer is brought into contact with the bladder in a heated state, and therefore the inner liner layer is adhered and bonded to the bladder. Thus, there arises a problem that, when the vulcanized tire is removed from the mold, the inner liner layer bonded to the bladder is taken away to the bladder side, resulting in the air-in phenomenon between the inner liner layer and the insulation or carcass.